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Monday, 8 December 2014

Electroreception in Mammals

The duck-billed platypus is one of the handful
mammals with the ability to sense electrical fields
By TwoWings, via Wikimedia Commons

Electroreception is the biological ability to perceive natural electrical stimuli or in simpler words, the ability to perceive the world via electricity.

Electroreception is quite common in aquatic or amphibious animals, since water is a much better conductor than air. Some quick examples include sharks, rays and the electric eel.

But how about mammals? With modern estimations putting the number of different mammalian species to at least 5400, there has to be a few possessing this extraordinary ability.

So, out of about 5400 species, the only mammals with the ability to sense electrical fields [from what we know] are the platypus (Ornithorhynchus anatinus) and echidnas (Tachyglossidae) from the monotreme order and the Guiana dolphin (Sotalia guianensis) from the cetacean order.

1. Platypus & Echidnas (Monotremes)
Monotremes are really, really bizarre. For instance, they are mammals and yet, they lay eggs! Today the only surviving monotremes are the platypus and four species of echidnas. And among other weird traits, they all possess electroreception.

Between the two, the duck-billed platypus has the most acute electric sense. The platypus has almost 40,000 electroreceptors arranged in a series of stripes along the bill, which allow it to catch large amounts of prey in murky waters at night while swimming with their eyes, ears and nostrils closed.

The species' electroreceptive system is highly directional, with the axis of greatest sensitivity pointing outwards and downwards. By making short-latency head movements called "saccades" when swimming, platypuses constantly expose the most sensitive part of their bill to the stimulus to localize prey as accurately as possible. Furthermore, it appears that electroreception is used along with pressure sensors to determine the distance to prey from the delay between the arrival of electrical signals and pressure changes in the water. Neat!

The platypus is also one of the handful mammals to produce venom.
The Venom is released by a calcaneus spur on each hind limb.
By Elonnon [GFDL], via Wikimedia Commons

As for echidnas, their electroreceptive abilities are much simpler. For instance, the western long-beaked echidna (Zaglossus bruijnii) possesses only 2,000 receptors whereas the short-beaked echidna (Tachyglossus aculeatus) barely has 400, concentrated in the tip of the snout.

This difference is probably due to the different habitat and feeding methods. Western long-beaked echidnas live in wet tropical forests where they feed on earthworms in damp leaf litter, so their habitat is probably favourable to the reception of electrical signals. Contrary to this is the varied but generally more arid habitat of the short-beaked echidna which feeds primarily on termites and ants in nests; the humidity in these nests presumably allows electroreception to be used in hunting for buried prey, particularly after rains. Experiments have shown that echidnas can be trained to respond to weak electric fields in water and moist soil. The electric sense of the echidna is hypothesized to be an evolutionary remnant from a platypus-like ancestor.

2. Guiana dolphin
Three years ago, researchers discovered that the Guiana dolphin (Sotalia guianensis) can also sense electrical fields. On the snout of the dolphin are rows of little holes, called hairless vibrissal crypts, originally associated with mammalian whiskers. Inside these crypts the dolphins have electroreceptors, capable of electroreception as low as 4.8 μV/cm, which is sufficient to help them detect small fish. Overall, their are as sensitive to electrical fields as platypuses are.

Although it is possible for other marine mammals to have also developed this ability, like the bottlenose dolphin, this has yet to be verified. However, many researchers expect that future research will reveal many other cetacean species with this ability.

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